Fixture-less bare board tester

ABSTRACT

Methods and devices for testing connectivity between connectors on a circuit board include utilizing a bias board having a photoconductive layer coated with a light-transmissive electrically conductive layer in conjunction with a light source and a voltage source to alternately charge and discharge conductors. A conductor discharged by connecting it to a ground via the bias board is determined to be electrically connected to a previously charged conductor if current flows between the conductor and the ground.

This application is a divisional of application Ser. No. 09/611,082filed Jul. 6, 2000, now U.S. Pat. No. 6,590,398.

FIELD OF THE INVENTION

The field of the invention is circuit test systems.

BACKGROUND OF THE INVENTION

As the spacing/pitch between connection pads on circuit boardsdecreases, previously utilized testing mechanisms such as the “bed ofnails” text fixture used for the electrical test of bare circuit boardsbecome inadequate. This inadequacy typically results, at least in part,from high cost, long delivery time, and/or insufficient density.

Although alternative testing mechanisms have been proposed, they too areat times inadequate or undesirable for testing high density/small pitchcircuit boards. One such method is described in U.S. Pat. No. 4,843, 329issued on Jun. 27, 1989 to Beha et al. (hereinafter “Beha”). Behadiscusses a method for use in detecting whether two pads areelectrically coupled (i.e. whether they are shorted together) by usingan optical beam to generate a charge on a first pad (and any other padconnected to it), irradiating the board with an optical beam to cause anelectron flux to be emitted from the second pad and a third pad, anddetecting and comparing the flux from the second and third pads todetermine if the second pad is electrically coupled to the first pad.This method is at times undesirable due to the need to causephotoemission of electrons and the need for testing within a vacuumchamber.

Another method is described in U.S. Pat. No. 5,357,194 issued on Oct.18, 1994 to Ullman et al. (hereinafter “Ullman”). Ullman discusses usinga series of contacts/probes coupled to a grid of light-transmissiveelectrically-conductive strips interconnected via photoconductive gateswherein a light source is utilized to selectively illuminate the gatesto provide power to selected connectors. In essence, Ullman simplyprovides a test device comprising a grid of contacts/probes and amechanisms for quickly selecting individual probes. As with the previous“bed of nails” approach, the Ullman method can become problematic whentesting high density circuit boards or boards on which the pads to notline up well with the probes. Although customized versions could beproduced to match up with specific circuit geometries, use of suchcustomized versions increases the cost and introduces a period of delaybefore the device is ready for use in testing a particular board.

Thus, there is a continuing need for new methods and devices for circuitboard testing, particularly in regard to devices and methods suitablefor low cost and timely testing of high density circuit boards.

SUMMARY OF THE INVENTION

The present invention is directed to methods and apparatus forfixture-less testing of circuits. More specifically, connectivitybetween any two conductive points on a circuit board is tested bycoupling one conductive point to a voltage source to charge theconductor, de-coupling the voltage source, and coupling the secondconductive point to a ground and measuring any current flow between thesecond conductive point and the ground to determine if the second pointwas charged via a connection to the first conductive point. The use oflight activated bias boards comprising a photoconductive layer coatedwith a light-transmissive electrically-conductive layer may beadvantageously used in coupling the conductors to the voltage source andground.

Various objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments of the invention, along with theaccompanying drawings in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a first method embodying the invention.

FIG. 2 is a symbolic perspective view of a first apparatus embodying theinvention.

FIG. 3 is a symbolic side view of a second apparatus embodying theinvention.

FIG. 4 is a perspective view of an embodiment of a bias board.

FIG. 5 is a perspective view of an alternative embodiment of a biasboard.

DETAILED DESCRIPTION

Referring to FIGS. 1-5, a method for testing a circuit board comprises:step 1, providing a circuit board 50 to be tested, the circuit boardhaving a first conductive element 51 and a second conductive element 52;step 2, establishing a voltage difference between the first conductiveelement 51 and a reference node 54 by coupling the first conductiveelement to a voltage source 200; step 3, decoupling the first conductiveelement 51 from the voltage source 200; step 4, electrically coupling anammeter 300 between the second conductive element 52 and the referencenode 54; step 5, utilizing the ammeter 300 to determine if a currentflows between the second conductive element 52 and the reference node54. If a current flows, the two conductive elements are electricallyconnected. If current doesn't flow, the two conductive elements are notelectrically connected.

Typically, the circuit board 50 to be tested will comprise patternedconductive layers on opposite sides of the circuit board, the patternedlayers including conductive elements such as pads and traces, and inparticular, conductive elements 51 and 52. In preferred applications,reference node 54 will comprise a grounded conductive element such as apad or trace electrically coupled to a ground plane of the circuit board50. As used herein, the phrase “circuit board” is intended to broadlycover any device comprising at least two electrically connectedconductive elements and thus includes but is not necessarily limited toprinted circuit boards, printed wiring boards, single and multi-layerinterconnects, and packaged and unpackaged integrated circuits. However,the methods and devices disclosed herein are contemplated as beingparticularly beneficial when the conductive elements to be tested arelocated on one or more substantially planar surfaces.

The disclosed method may be advantageously employed in conjunction witha test system designed to implement the disclosed method. A preferredsuch system comprises: a light source 400; a voltage source 200, anammeter 300, and a bias board 100 comprising at least one bias member110 having a photoconductive layer 112 coated with a transparent ortranslucent conductive layer 114, wherein each bias member 110 iselectrically coupled to every other bias member 110 such that a voltageestablished on the photoconductive layer 112 of one bias member resultsin an approximately equivalent voltage being established on thephotoconductive layer 112 of every other bias member 110.

In a preferred test system, bias board 100 comprises a single biasmember 110 which in turn comprises a single photoconductive layer 112coated with a single light-transmissive electrically-conductive layer114. It is contemplated that the use of photoconductive layer 112 havingone surface completely coated with a light-transmissiveelectrically-conductive layer 114 provides the greatest flexibility inapplying the test system to boards of varying geometries. With such abias board, a voltage applied to the conductive layer 114 can be appliedto any conductor in contact with the photoconductive layer 112 byilluminating a point on the photoconductive layer adjacent to theconductor. As there are no gaps in either the conductive layer 114 orthe photoconductive layer 112, and as there are nonon-light-transmissive conductors obscuring any part of the bias board,there is no need to reconfigure the bias board or carefully align it tomatch up with the geometry of the board to be tested. In less preferredembodiments, bias board 100 may comprise a plurality of bias members110. A bias board 100 having a plurality of bias member 110 pushed upagainst each other so as to minimize or eliminate any gaps between thebias members would generally be almost as flexible as a board comprisinga single bias member and may actually be more flexible in regard totesting circuit boards which are larger than the size of a single biasmember if the maximum size of such a member is smaller than the size ofthe board to be tested. Bias boards 100 having bias members separated bya distance D are contemplated as being less desirable than a boardhaving a single member is adequate where such an arrangement stillpermits contact with the conductors to be tested. Photoconductive layers112 may typically be characterized as having a coated side 112 a and acircuit contacting side 112 b. For the preferred embodiment utilizing asingle bias member 110 the photoconductive layer 112 will have an areagreater than equal to the area of the circuit board to be tested.Whether single or multiple bias members 110 are used in bias board 100,it is contemplated that photoconductive layers 112 may advantageouslycomprise circuit contacting surfaces which have an area greater or equalto A square inches wherein A is one of 0.5, 1, 2, 4, 9, 16, or 25. It isalso contemplated that when testing circuit boards comprising aplurality of contact pads, the circuit contacting surface willadvantageously have a minimum width of at least X times the maximumpitch between contact pads where X is one of 1, 2, 5, or 10.

Although transparent or translucent layer 114 may comprise any lighttransmissive material or combination of materials it is contemplatedthat layers comprising tin oxide or indium oxide may be advantageouslyused. Similarly, although any photoconductive material or combination ofmaterials may be used for photoconductive layer 112, it is contemplatedthat layers comprising cadmium sulfide or zinc sulfide may beadvantageously used.

It is contemplated that voltage source 200 and ammeter 300 may becoupled to the light-transmissive conductive layer via one or moreswitches 500 such that while the voltage source is electricallyconnected to the conductive layer the ammeter is not and that thevoltage source is not electrically connected to the conductive layerwhile the ammeter is electrically coupled to the conductive layer.

For circuit boards having conductive element on opposing sides, a systemcomprising a second bias board and a second light source is preferred.In such a system the two bias boards are positioned between the twolight sources, and the transparent or translucent conductive layer ofthe second bias board is positioned between the second light source andthe photoconductive layer of the second bias board. When arranged inthis fashion, a circuit board can be positioned between the two biasboards such that the circuit contacting surfaces of the bias members maybe placed in contact with elements 51 and 52 of circuit board 50. Analternative, less desirable approach to dealing with conductive elementson opposite sides of a circuit board is to flip the circuit board overafter the first conductor has been charged so that a single bias boardcan be used to both charge and discharge conductors located on oppositesides of the circuit board.

Any light source capable of providing a light which can be focused andpositioned to illuminate small enough points on the photoconductivelayer may be utilized. However, it is contemplated that typicalembodiments will comprise a light bulb or laser 410 and a focusing optic420 such as a lens. Alternative embodiments may utilize differingnumbers or arrangements of lights and optical elements for positioningand directing the light provided. As an example, a system utilizing asingle light source 400 but two bias boards 100 can be used toilluminate both boards through the proper direction of the generatedlight. Alternatively, multiple light sources may be used for one or morebias boards.

When the disclosed methods and apparatus are combined, the step ofcoupling the first conductive element to the voltage source involvesplacing it in contact with a photoconductive layer of a bias elementwhich is electrically coupled to the voltage source via theelectrically-conductive light-transmissive layer, and shining a light onthe photoconductive layer at a point adjacent to the first conductiveelement. Similarly, the step of coupling the second conductive elementto the ammeter involves placing it in contact with a photoconductivelayer electrically coupled to the ammeter via the electricallyconductive light transmissive layer, and shining a light on thephotoconductive layer at a point adjacent to the second conductiveelement. Such a method may be described as including the followingsteps: providing a circuit board to be tested; providing a first biasboard and placing the first bias board in contact with a first surfaceof the circuit board; providing a second bias board and placing thesecond bias board in contact with a second surface of the circuit boardwherein the second surface is opposite the first surface; coupling thefirst bias board to a voltage source and causing light from a lightsource to contact the first bias board adjacent to a first conductor ofthe circuit board; preventing light from the light source fromcontacting the first bias board; coupling the second bias board to aground via an ammeter and causing light from a light source to contactthe second bias board adjacent to a second conductor of the circuitboard.

Causing the light from the light source(s) to contact thephotoconductive layer of the bias board may be accomplished in a numberof ways, including but not necessarily limited to stepping or moving thecircuit board under test with respect to the light source and/orscanning the circuit board under test by deflecting the light source.

From the foregoing it can be seen that the disclosed devices and methodsprovide numerous advantages over the prior art. One such advantage isthat no modification of the test system is necessary to handle testingof circuit boards having different configurations. Another is thateliminating the need for mechanical movement and alignment of a proberesults in a significant time savings. Yet another is that potentiallydamaging contact between a probe and conductive elements is eliminated.Still another is that testing can be performed at room temperature andpressure. Although other advantages exist, any one of the listedadvantages by itself makes for a testing system which permits lower costand more timely testing of high density circuit boards.

Thus, specific embodiments and applications of devices and methods fortesting circuit boards have been disclosed. It should be apparent,however, to those skilled in the art that many more modificationsbesides those already described are possible without departing from theinventive concepts herein. The inventive subject matter, therefore, isnot to be restricted except in the spirit of the appended claims.Moreover, in interpreting both the specification and the claims, allterms should be interpreted in the broadest possible manner consistentwith the context. In particular, the terms “comprises” and “comprising”should be interpreted as referring to elements, components, or steps ina non-exclusive manner, indicating that the referenced elements,components, or steps may be present, or utilized, or combined with otherelements, components, or steps that are not expressly referenced.

What is claimed is:
 1. A test system for testing a circuit board havinga first conductive element and a second conductive element, the systemcomprising: a light source; a bias board comprising at least one biasmember having a photoconductive layer coated with a transparent ortranslucent conductive layer, wherein each bias member is electricallycoupled to every other bias member such that a voltage established onthe photoconductive layer of one bias member results in an approximatelyequivalent voltage being established on the photoconductive layer ofevery other element.
 2. The test system of claim 1 further comprising: avoltage source; an ammeter that electrically couples the secondconductive element and a reference node; and at least one switchintermittently electrically coupling the voltage source and ammeter tothe bias board.
 3. The test system of claim 2 wherein the bias boardcomprises a single bias member.
 4. The test system of claim 3 whereinthe circuit to be tested comprises a plurality of contact pads having amaximum pitch between contact pads, and wherein the bias membercomprises a first surface coated with the transparent or translucentconductive layer and a second, circuit contacting surface opposite thefirst surface, wherein the circuit contacting surface has a minimumwidth of at least X times the maximum pitch between contact pads where Xis one of 1, 2, 5, or
 10. 5. The test system of claim 4 wherein thecircuit contacting surface has an area greater or equal to A squareinches wherein A is one of 0.5, 1, 2, 4, 9, 16, or 25.